1. Field of the Invention
The present invention relates to a disc signal reproducing method adapted for use in reading data from an optical disc which is driven for playback at a constant angular velocity.
2. Description of the Prior Art
In an optical disc apparatus for reading data from an optical disc by irradiating a laser beam thereto and causing intensity changes or phase changes of the reflected light, there exists a disadvantage that, in a high density recording mode, a bit rate (space frequency of minimum bit) recorded on tracks of the optical disc is raised to consequently narrow a phase margin (detection window margin) in a data detector of a playback system.
Particularly in a CAV (constant angular velocity) disc where the rotation rate is kept constant, the bit rate of data recorded on an inner track Tin of the disc D is higher than that on an outer track Tout correspondingly to the radii r1 and r2 thereof as illustrated in FIG. 7, so that the inter-code interference of the reproduced data becomes greater toward the innermost portion of the disc.
Therefore, when a playback advances from an outer track to an inner track, the eye pattern of a reproduced RF signal is changed to be narrower as illustrated in FIGS. 8a, 8b and 8c, and the data error rate detected from such eye pattern tends to increase.
In FIG. 9, a solid line (A) represents the trend of a phase margin (detection window margin) in detecting digital data obtained by first recording NRZ data, which is recordable with a relatively high density, at one clock rate on an optical disc whose rotation angle is constant, and then digitizing the reproduced RF signal by a binary detector. In the graph of FIG. 9, the abscissa represents the recording density by the pit length .lambda. (.mu.m) of a minimum bit Pmin, and the ordinate represents changes in the phase margin of a clock signal when detecting the binary data from the RF signal on the basis of a threshold level determined at the center of the eye pattern.
As will be understood from the graph, on the outer track where the minimum bit interval of the data is about 1 micron, the eye pattern of the reproduced RF signal is such as illustrated in FIG. 8a, and the phase margin for detection is about 0.8 which indicates a considerable allowance. However, when the minimum bit interval becomes close to 0.5 micron on the inner portion of the optical disc, the eye pattern of the reproduced RF signal is so changed as illustrated in FIG. 8c, whereby the phase margin for detection is sharply reduced to be smaller than 0.5. (The phase margin signifies an allowable range of the detection window phase for maintaining the data error rate under a predetermined value in a playback operation.)
Consequently, for reproducing the signal in a manner to maintain the data error rate below 10.sup.-7 or so during playback of the disc where the angular velocity is constant as mentioned, it is extremely severe for the detector to retain a sufficient phase margin on the inner portion of the disc, hence necessitating strict quality control of the optical disc and bringing about an increased cost in the playback circuit.
Furthermore, since the recording capacity of the optical disc as a whole is limited by the recording density on the inner portion, the data quantity recordable on a single optical disc is restricted by the recording surface density on the inner portion and is therefore not increasable to a sufficiently great value.